1. Field of the Invention
This invention relates to a method of determining the direction of arrival of a radio signal distorted by multipath and being received by an antenna array having a plurality of radiating elements, as set forth in the preamble of claim 1. The invention further relates to a radio base station using this method and to a radiocommunications system containing such a base station, as set forth in the preambles of the respective independent claims.
2. Description of the Related Art
U.S. Pat. No. 4,965,732 discloses a method of determining the direction of arrival of a radio signal distorted by multipath. The direction of arrival is determined using the so-called ESPRIT algorithm (ESPRIT: Estimation of Signal Parameters using Rotational Invariance Techniques). The radio signals received by an antenna array are evaluated as follows. First, data are collected for each radiating element which specify the receive level and the phase position. From these data, two matrices are formed, namely the so-called auto-covariance matrix and the so-called cross-covariance matrix. Next, by eigen-decomposition of the two matrices, the number of signal sources and the noise variance are determined, from which the so-called subspace rotation operator can be computed, which contains information on the direction of arrival (see U.S. Pat. No. 4,965,732, at the bottom of column 10). The decomposition of the matrices into their eigenvalues ("eigen-decomposition") is a computationally intensive operation, so that a powerful computer is needed to carry out the method. Since the method is used within a base station which is to receive radio signals from different directions by means of an antenna array, the cost and complexity of the base station are very high.
In an article by S. C. Swales et al, "The Performance Enhancement of Multibeam Adaptive Base-Station Antennas for Cellular Land Mobile Radio Systems", IEEE Transactions on Vehicular Technology, Vol. 39, No. 1, February 1990, pages 56-67, a base station is described which has an adaptive antenna array, a "smart" antenna. To steer the radiation pattern, use is made of a beamformer. The article deals essentially with the design of a cellular radio network in which base stations with such adaptive antenna arrays are used to improve the utilization of the radio resources. This is achieved by reusing one and the same radio frequency for different radio channels in several directions. To avoid interference between neighboring base stations, network planning is carried out with the aid of the "co-channel reuse ratio Q" (see page 59, left column). Nothing is said there about the pointing of the radiation patterns and the required determination of the direction of arrival.
An article by J. S. Thompson, "Smart Antenna Arrays for CDMA Systems", IEEE Personal Communications, October 1996, pages 16-25, describes antenna arrays in which a method of determining the direction of arrival of radio signals is carried out using the ESPRIT algorithm. Accordingly, the computational complexity of this method, too, is very large.